ZyXEL Communications USG40 User Manual page 370

Chapter 19 IPSec VPN
A
Y
X
Most routers like router A now have an IPSec pass-thru feature. This feature helps router A
recognize VPN packets and route them appropriately. If router A has this feature, router X and
router Y can establish a VPN tunnel as long as the active protocol is ESP. (See Active Protocol on
page 371 for more information about active protocols.)
If router A does not have an IPSec pass-thru or if the active protocol is AH, you can solve this
problem by enabling NAT traversal. In NAT traversal, router X and router Y add an extra header to
the IKE SA and IPSec SA packets. If you configure router A to forward these packets unchanged,
router X and router Y can establish a VPN tunnel.
You have to do the following things to set up NAT traversal.
• Enable NAT traversal on the ZyWALL/USG and remote IPSec router.
• Configure the NAT router to forward packets with the extra header unchanged. (See the field
description for detailed information about the extra header.)
The extra header may be UDP port 500 or UDP port 4500, depending on the standard(s) the
ZyWALL/USG and remote IPSec router support.
X-Auth / Extended Authentication
X-Auth / Extended authentication is often used when multiple IPSec routers use the same VPN
tunnel to connect to a single IPSec router. For example, this might be used with telecommuters.
In extended authentication, one of the routers (the ZyWALL/USG or the remote IPSec router)
provides a user name and password to the other router, which uses a local user database and/or an
external server to verify the user name and password. If the user name or password is wrong, the
routers do not establish an IKE SA.
You can set up the ZyWALL/USG to provide a user name and password to the remote IPSec router,
or you can set up the ZyWALL/USG to check a user name and password that is provided by the
remote IPSec router.
If you use extended authentication, it takes four more steps to establish an IKE SA. These steps
occur at the end, regardless of the negotiation mode (steps 7-10 in main mode, steps 4-7 in
aggressive mode).
Certificates
It is possible for the ZyWALL/USG and remote IPSec router to authenticate each other with
certificates. In this case, you do not have to set up the pre-shared key, local identity, or remote
identity because the certificates provide this information instead.
ZyWALL/USG Series User's Guide
370